The invention relates generally to the selection of that cell where a certain cellular system terminal operates at any given time. Particularly the invention relates to the optimisation of the cell selection and relection in a system, which has cells providing different features relating to the transmission of information, and in which the terminals have different abilities to utilise these features.
A cellular radio system comprises stationary base stations, each having a certain coverage area, and terminals which can move in relation to the base stations and their coverage areas. The coverage areas are also called cells. In this patent application a mobile phone is treated as an exemplary terminal. When a particular mobile phone is switched on, it performs a cell selection. In other words it somehow tries to find the best received base station signal and tries to register with the so called location area (LA) which represents this base station. Registering means that the mobile phone notifies the mobile network through the base station that it can receive calls via that location area, with which said base station is associated. In the idle mode a mobile phone regularly receives messages transmitted by the base station in order to detect paging messages, which represent an incoming telephone call, and other messages intended for this mobile phone. At the same time the mobile phone monitors the power of signals transmitted by other adjacent base stations, so that it rapidly can change the base station, when required. Changing the base station in idle mode or during some form of packet-switched connections is called cell reselection. Changing the base station during a circuit-switched connection is known as a handover.
In prior art cellular systems the base stations are mutually substantially similar concerning the data transmission features, i.e. a mobile phone receives the same service from the network, regardless of in which cell it operates. An exception is formed by some cell priority arrangements, with which the mobile phones are controlled to operate in cells belonging to a particular home area, or to avoid certain so called handover cells. In the home area arrangement a geographically fixed home area is defined for a mobile phone by an agreement between the user and the operator maintaining the cellular system. When a mobile phone operates in the home area cells it will get a reduction on the common call charges, or other benefits.
In prior art cellular systems also the terminals have data transmission features which are very close to each other. Usually only one type of air interface is defined in the system, whereby the air interface definitions relate to the timing of transmission and reception, the available frequencies, the transmission rate, i.e. the bit rate used in the data transmission, and other corresponding factors.
The actions for selecting a cell in the GSM system (Global System for Mobile telecommunications) and in its extension DCS1800 (Digital Communications System at 1800 MHz) are defined in the EBU (European Broadcasting Union) and ETSI (European Telecommunications Standards Institute) standards ETS 300 535 (GSM 03.22) and ETS 300 578 (GSM 05.08). The actions for selecting a cell relating to a packet switched GPRS arrangement (General Packet Radio Service) are defined in the draft standard GSM 03-64. On a general level the same considerations can be applied also in other digital cellular systems. Cell selection is also discussed in the Finnish patent application no. FI-970855 (Nokia Mobile Phones Oy). Cell selection or reselection can also be called selection of the base station, because a certain base station is responsible for the radio communications in each cell. In addition to the GSM system the present invention can find application for instance in a cellular system according to the IS-136 standard (Interim Standard 136) and in the planned UMTS system (Universal Mobile Telecommunications System).
There are four requirements on the cell so that a mobile phone could camp in it normally:
the cell must belong to that operator""s network, which is selected to be used,
the network must not have barred the cell,
the location area represented by the cell must not belong to the list of forbidden location areas defined for each mobile phone, and
the attenuation on the radio path between the mobile phone and the base station must be lower than a certain threshold value determined by the operator (this requirement is for short called the path loss criterion).
A cell which meets the above mentioned requirements is called a suitable cell.
When the mobile phone has been switched on it runs through the so called BCCH (Broadcast Control CHannel) signals which it has received, in their order of strength, and settles to operate in that suitable cell, which has the strongest signal. The BCCH signal can also contain a recommendation value related to the cell, which indicates whether the cell is a suitable cell or not from the system point of view. The mobile phone settles to operate in a non-suitable cell only if there are no available suitable cells. This phase is called cell selection. The present invention is basically applicable to both cell selection and reselection, which can be jointly designated as xe2x80x9cselecting a cellxe2x80x9d.
A mobile phone will regularly examine whether there is a suitable cell in the neighbourhood, which regarding the radio connection would be more advantageous than the current cell, and when required it makes a cell reselection. The mobile phone can make a reselection due to three alternative reasons:
the new cell is better than the current cell according to certain cell reselection criteria,
some of the characteristics of the current cell changes so that the cell is not anymore a suitable cell, but a new cell is suitable, or
the mobile phone detects that the downlink signalling connection in the current cell is disconnected.
The cell selection and cell reselection are based on two parameters calculated by the mobile phone, the so called C1 and C2 parameters, which are defined in the standard ETS 300 578 (GSM 05.08). Of these the first or the C1 parameter represents the power level received by the mobile phone from the considered base station, compared to the minimum value of the received power level and to the allowed maximum transmission power level of the mobile phone, which are defined in the system. The value of the C2 parameter is affected by the value of the C1 parameter and by two correction factors, of which the first one is an offset parameter transmitted by the base station, and the second is a delay intended to prevent rapid consecutive cell reselections by the mobile phone. Further the so called C31 and C32 parameters are used in the GPRS system, which parameters in other respects correspond to the C1 and C2 parameters, but the calculation of them utilises initial values, which are characteristic for hierarchic cell structures and for packet switched connections.
In order to increase the transmission capacity in digital cellular systems the trend is towards more effective modulation methods than the current ones, at least regarding the data channels. In an enhanced form of the GSM system the object is to advance from the GMSK modulation (Gaussian Minimum Shift Keying) to a modulation method which enables a higher bit rate. Below the QAM modulation (Quadrature Amplitude Modulation) is used as an example of a modulation method of this type; alternative embodiments of the QAM modulation are at least the B-O-QAM and the Q-O-QAM (Binary Offset QAM, Quaternary Offset QAM), of which the first one is in a way a subset of the second one. Another suitable new modulation method is CPM (Continuous Phase Modulation), of which a possible embodiment is the xcfx80/4-DBCPM (Differentially encoded Binary CPM). In prior art there are not any known methods or devices which would take into account the effects of modulation changes on cell selection.
The object of the present invention is to present a method for cell selection and/or reselection, which takes into account the ability of the base stations and the terminals to use different modulation methods. An object of the invention is also to present a base station and a terminal for realising in practice the method according to the invention.
The objects of the invention are attained by including in the general information transmitted by the base station also information about the modulation methods supported by the base station and possibly also by other base stations surrounding it, whereby the terminal can select a new cell so that the base station of the new cell supports the modulation required by the terminal.
The method according to the invention is characterised in that it comprises steps, in which
the terminal is notified of which modulation methods are supported by those base stations with cells, which are possible new cells of the terminal, and
a new cell for the terminal is selected by utilising information about which modulation methods are supported by those base station with cells, which are possible new cells of the terminal.
The invention relates also to a base station, which is characterised in that it comprises means for including information about which modulation methods the base station supports in the control information transmitted to the terminals. Further the invention relates to a cellular system terminal, which comprises means for receiving information from the base station and means for generating and processing comparison information representing several base stations for the cell reselection, and which is characterised in that it comprises means for including information about which modulation methods the base station support in the generation of said comparison information.
The base station transmits information on a so called broadcast control channel to all terminals operating in the cell. This information can include information about which modulation methods the base station supports. When the terminal selects a possible new cell it places the possible new cells in an order of preference, which is affected by the signal strength received from their base stations and by the information about which modulation methods each base station supports. The terminal tries to select as a new cell that cell, where the base station is received better than a certain target level, and which further can operate with that modulation, which from the terminal""s point of view is currently the most advantageous. In the case of a packet switched radio connection the selection made by the terminal can also be affected by how high transmission capacity the new base station can provide. If the terminal must select a base station, which supports one or more modulation methods, which do not represent an optimum regarding the terminal, then the terminal tries to change to a more optimal cell as soon as possible. The preference for optimal cells regarding the modulation methods is most simply realised by including a factor representing the modulation methods in the calculation formula representing the preference order.